1. Field of the Invention
The present invention relates to a solid polymer electrolyte membrane which enhances thermostability. In particular, the present invention relates a membrane electrode assembly for solid polymer electrolyte fuel cells having a polymer electrolyte membrane that improves the stability of a sulfonic acid group and power durability at high temperature when the membrane is employed as a proton conductive membrane contained in fuel cells.
2. Related Art
Fuel cells generate electric power by a process in which hydrogen gas, produced from various hydrocarbon fuels, such as natural gas, methane, and oxygen in the air, are electrochemically reacted to generate electric power directly. Therefore, they have been attracting attention as non-polluting power generating systems with higher efficiency, that can directly convert the chemical energy in fuels into electric energy.
A solid polymer electrolyte membrane-electrode assembly is typically employed in a fuel cell, in which the assembly is typically constructed of a pair of fuel electrode catalyst-supporting electrode membranes, which are a fuel electrode and an air electrode, as well as a proton-conductive electrolyte membrane (hereinafter sometimes referred to as a “solid polymer electrolyte membrane”) that is disposed between the electrode membranes. The hydrogen gas turns into hydrogen ions and electrons by the action of the catalyst on the fuel electrode, and then the hydrogen ions travel through the solid polymer electrolyte membrane to be converted into water at the air electrode by a reaction with oxygen.
In recent years, fuel cells have been desired that provide a higher power generating performance. In order to enhance power generating, it is desired that the fuel cell be used at high temperatures. Therefore, it is desirable that the solid polymer electrolyte membrane-electrode assembly and be able to operate under a broader range of conditions, in particular, it is desirable that the membrane have higher proton conductivity at higher temperatures.
A polymer with a sulfonic acid group has usually been employed as the solid polymer electrolyte membrane, so as to satisfy the demands. In addition, the applicant suggests that a specific polymer having a sulfonic acid group be used as the proton conductive membrane that exhibits higher proton conductivity (see Patent Documents 1 to 3).
Patent Document 1: Japanese Unexamined Patent Application Laid-Open No. 2004-345997
Patent Document 2: Japanese Unexamined Patent Application Laid-Open No. 2004-346163
Patent Document 3: Japanese Unexamined Patent Application Laid-Open No. 2004-346164
However, there are problems with the conventional solid polymer electrolyte membrane formed from a polymer having a sulfonic acid group, in that a reversible elimination reaction is likely to occur on the sulfonic acid group or a cross-linking reaction may progress due to higher temperatures, which tends to decrease proton conductivity or embrittle the membrane, resulting in a possible decrease in the power output of the fuel cell or a shutdown in power generation due to a rupture in the membrane. In order to reduce the probability of these problems as much as possible, currently, the fuel cell is operated below a certain maximum temperature, which consequently results in a power generation output limit.
Accordingly, the solid polymer electrolyte membrane-electrode assembly in which a solid polymer electrolyte membrane exhibits superior thermal resistance while maintaining the proton conductivity at the prior level is desirable.